Pruning summer lilac: The right way to do it

butterfly on summer lilac
butterfly on butterfly bush

The summer lilac is one of the most magnificent flowering shrubs and a butterfly magnet in the garden. Here you can read how to prune the flowering shrub to increase the abundance of flowers.

The summer lilac (Buddleja davidii), also called butterfly bush, is an undemanding flowering shrub that grows in any soil that is not too heavy. It should not be missing in any flower garden, on the one hand because of its long flowering period from July until far into the fall, and on the other hand because its nectar-rich flowers magically attract butterflies. The latter characteristic has also earned the summer lilac the name butterfly bush. To ensure that it blooms lushly, however, the summer lilac should be pruned regularly.

Like almost all summer-flowering woody plants, the summer lilac also forms its flowers on the so-called new wood. This means that the shoot, which is still in bud in late winter, already bears inflorescences in summer at the end of the shoot and also at the ends of its short side shoots.

When and how to cut summer lilac?

Pruning can significantly increase the abundance of flowers of the summer lilac. It is best to prune the shrub in winter, or more precisely, the ideal time is late winter. If you cut back all the flowering shoots from the previous year, the butterfly bush will sprout vigorously and form long new shoots with particularly large flower corollas. You will achieve the maximum flower size if you leave only one short stub with two buds on each of the previous year’s shoots.

However, the disadvantage of annual heavy pruning should not be concealed: Over the years, dense, overgrown branch whorls develop on the plants, which must be thinned out regularly. If a homogeneous crown structure is more important to you than large flowers, you should vary the cutting heights, i.e. cut back some shoots more and shorten other, favorably placed branches by only a third.

After pruning, the butterfly bush must sprout anew from its so-called dormant buds. This feat takes more time than a normal sprouting, and therefore the flowering starts correspondingly later. To prevent the flowering period from shifting too far into late summer, it is recommended to prune the woody plants by mid-February at the latest. If possible, reach for the pruning shears on a frost-free day so that the already quite brittle wood of the summer lilac does not splinter when cut. If it gets cold again after pruning, that’s no big deal: summer lilac is, especially on nutrient-poor sandy soils, tougher than is commonly assumed.

Step by step: properly prune summer lilac

Summer lilac before cutting

The summer lilac remains green in mild winters. In February, when there is no longer a threat of heavy frost, it is time to vigorously prune the long, faded shoots of the summer lilac. If you prune the branches earlier, there is a risk that the fresh shoots will freeze. A later pruning, on the other hand, shifts the summer flowering time further back.

Determine competing shoots

Competing shoots, as well as branches that are too close together, are removed. Here, the shoots are so dense that they hinder each other in growth. The shrub could become bare over time.

Thinning out summer lilac

Directly at the base, one of the two shoots that are too dense is cut off with pruning shears. In this way, the summer lilac is thinned out and the remaining branches can develop much better again.

Pruning below the leaf settings

The deep cut below the leaf settings prevents the branch from resprouting in the spring. If the competing shoot is too strong for the shears, remove it with a handy folding saw instead.

Shorten flower shoots

Use pruning shears to vigorously cut back all flower shoots from the previous year. They are cut off above two opposite leaf bases. As a rule, leave at least one and no more than three of the opposite leaf bases. This is where new shoot buds form, which will sprout vigorously again in the spring and bear new, long flower corollas in the summer.

Cut off weak side shoots

Thin side branches of the summer lilac are completely removed, as well as those annual shoots that crowd others or cross with them.

Summer lilac after pruning

After the work is completed, the summer lilac consists only of a low basic structure. When budding, the shrub now puts all its energy into the remaining branches. By summer it is again as tall as a man and bears numerous large inflorescences at the ends of the new shoots.

How to prune the yellow summer lilac?

The yellow summer lilac (Buddleja x weyeriana ‘Sungold’) is not completely hardy, but with good winter protection in mild regions it can survive in the garden. Like Buddleja davidii, it blooms on new wood and is therefore also pruned vigorously in spring. To be on the safe side, wait until the strongest frosts have passed before pruning in spring. The optimal pruning date for outdoor plants is mid-May. If you keep the yellow summer lilac in a tub on the terrace, however, you can prune it as early as February. If there is a threat of even stronger frosts, simply bring the plant back indoors or into the garage at short notice.

How to prune the alternate-leaved butterfly bush?

The alternate-leaved butterfly bush (Buddleja alternifolia) shows a completely different growth character, because it forms numerous small flower clusters in the leaf axils. However, the fact that it bears its flower buds on the previous year’s shoots is decisive for the pruning technique. If it were pruned vigorously in the spring, like the summer lilac, you would have to do without the flowers in the summer. For this reason, the alternate-leaved butterfly bush is usually rarely pruned.

If the shrub becomes too dense or the bloom diminishes over time, pruning with a sense of proportion is nevertheless not a problem. You should only lightly thin out the evergreen in late winter if necessary, by removing the oldest shoots directly at the point of attachment. In this way, the shrub is rejuvenated, sprouts vigorously again and blooms more abundantly in the coming years.

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